Field of the Invention
The present invention relates to a method for preparing aromatic carboxylic acids from alkylaromatics by liquid-phase oxidation. More particularly, the present invention relates to a method for preparing aromatic carboxylic acids from alkylaromatics by oxidation in acetic acid as solvent with oxygen-containing gas in the presence of cobalt/manganese/bromine complex catalyst, wherein nickel and carbon dioxide are added in an appropriate amount to increase an activity of cobalt/manganese/bromine complex catalyst. Especially nickel has a synergistic effect with carbon dioxide and maximize the formation of the desired product having the corresponding carboxylic groups to the number of alkyl groups in a reactant.
The first liquid-phase oxidation in place of vapor-phase oxidation used in a preparation of aromatic carboxylic acid was introduced in U.S. Pat. No. 2,245,528 to perform at 100-320xc2x0 C. under the pressure to keep a saturated fatty acid solvent in liquid state and in the presence of metal catalyst having several valances. An activity was the most with cobalt among metals and accelerated by adding ketones or aldehydes. However, this method converts only one alkyl group of mono-, di-, or trimethyl benzene to benzene monocarboxylic acids such as benzoic acid, toluic acid, and dimethyl benzoic acid.
Other liquid-phase oxidations of alkyl aromatics at an elevated temperature and pressure and in the presence of catalyst have been disclosed to convert all the alkyl groups to the corresponding carboxylic acids. Used catalysts are combinations of bromine and transition metals, especially use of cobalt/manganese/bromine complex catalyst in the oxidation of p-xylene to terephthalic acid (U.S. Pat. No. 2,833,816). Further, preparations of benzene di or tricarboxylic acid from di or trimethyl benzene such as p-xylene, m-xylene or pseudocumene (1,2,4-trimethyl benzene) by oxidation have been developed and widely commercially applied (U.S. Pat. Nos. 5,041,633 and 5,081,290). The prepared aromatic carboxylic acids after purified have been using as raw materials to produce polyesters, fibers, films and the like.
However, the use of cobalt/manganese/bromine complex catalyst has some drawbacks in side-reactions, expensive cost, difficulties in treatment and sedimentation. Since reduced reaction time to produce aromatic carboxylic acids will ensure an improvement of productivity and manufacturing cost, development of efficient catalysts and processes has been constantly progressing. Even now, there is no better catalyst than cobalt/manganese/bromine complex catalyst.
Development has been continued to improve an activity of a catalyst by adding other components. Nickel has been used in liquid-phase oxidation of dimethyl benzene or pseurocumene with molecular oxygen but cobalt was not added (U.S. Pat. No. 4,786,753). Nickel has been also used in liquid-phase oxidation of p-xylene with peroxide in the presence of cobalt/manganese/bromine complex catalyst (KR Patent 2000-41505) but the activity was worse than that of cobalt/manganese/bromine complex catalyst.
On the other hand, since liquid-phase oxidation of alkyl aromatics in the presence of a stoichiometric excess of oxygen- or highly pure oxygen-containing gas is susceptible to explosion for formation of flammable gas, carbon dioxide is used to reduce a risk of flammability or explosion. It is further known that it does not affect the activity of the reaction (U.S. Pat. No. 5,693,856 and EP Patent 0785183 A2). Recently carbon dioxide has been used with cobalt/manganese/bromine complex catalyst (KR Patent Publication 2000-67444) and additionally with alkali metal or alkali earth metal (KR Patent Publication 2000-41507) to improve the reaction efficiency in the preparation of aromatic carboxylic acid via liquid-phase oxidation. However, the development of metal or non-metal component to be combined optimally with carbon dioxide is highly demanded to improve the reaction efficiency largely.
The present invention has been completed with the development of optimal combination of carbon dioxide and nickel, which exhibits synergistic effect, in the preparation of aromatic carboxylic acids by liquid-phase oxidation in the presence of cobalt/manganese/bromine complex catalyst to remarkably improve reaction efficiency due to increase in the reaction rate of the oxidation.
An object of the present invention is to provide a method for preparing aromatic carboxylic acids, which is highly efficient in the presence of commercial cobalt/manganese/bromine complex catalyst with carbon dioxide and nickel.
In the preparation of aromatic carboxylic acids by liquid-phase oxidation of alkylaromatics and partially oxidized intermediates with oxygen-containing gas in the presence of cobalt/manganese/bromine complex catalyst and in acetic acid as the solvent, the present invention is characterized in that nickel and carbon dioxide are used in the preparation of aromatic carboxylic acids. Especially, nickel and carbon dioxide enhance each other""s promotional effect, showing namely synergistic effect.
The present invention is described in detail as set forth hereunder.
The present invention uses an appropriate amount of carbon dioxide and nickel in the conventional liquid-phase oxidization of alkylaromatics which is performed by using oxygen-containing gas in the presence of cobalt/manganese/bromine complex catalyst and in acetic acid as the solvent to produce aromatic carboxylic acids, thus provides some advantages in that the reaction efficiency with increases in the reaction rate and reaction temperament is highly improved, and the selectivity toward the product polycarboxylic acids is largely increased due to a sharp decrease in the formation of partial oxidized intermediates.
The present invention is described in more detail in accordance with addition components and reaction conditions as set forth hereunder. The addition components are alkylaromatics, oxygen-containing gas, cobalt/manganese/bromine complex catalyst, and reaction activators of nickel and carbon dioxide.
The alkylaromatics are the aromatic compounds having at least one alkyl group. Examples are toluene, o-xylene, m-xylene, p-xylene, pseudocumene (1,2,4-trimethylbenzene), mesitylene (1,3,5-trimethylbenzene), durene (2,3,5,6-tetramethylnaphthalene), methylnaphthalene, 2,6-dimethylnaphthalene, 4,4xe2x80x2-dimethylbiphenyl and an intermediate thereof. These alkylaromatic compounds are converted to the corresponding aromatic carboxylic acids, benzoic acid, phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, trimesic acid, pyromellitic acid, carboxylnaphthalic acid, 2,6-dicarboxynaphthalic acid and 4,4xe2x80x2-dicarboxybiphenylic acid.
It is prefer to use manganese/cobalt in the atomic weight ratio range of 0.1-5, preferably 0.5-3 in cobalt/manganese/bromine complex catalyst. It is further prefer to use bromine/(manganese+cobalt) in the atomic weight ratio range of 0.1-5, preferably 0.5-2. Cobalt is used 50-10,000 ppm relative to the total amount of reactants, preferably 100-1,000 ppm. Any bromine compound such as HBr, Br2, tetrabromoethane and benzyl bromide may be used as a source of bromine. As sources of manganese and cobalt, any compound being soluble in a used solvent may be possible (e.g. acetates, carbonates, acetate tetrahydrates and bromides). It is prefer to have the atomic weight ratio of nickel/manganese in the range of 0.01-1. If the weight ratio is higher which means more amount of nickel than preferred is used, the other catalyst compounds could not act as the catalyst.
Reaction gas used in the present invention is an oxygen or a mixture gas of oxygen and inert gas such as nitrogen. Amount of oxygen is in the range of 2-75% (v/v) to the total gas amount. Carbon dioxide is also used to activate the reaction efficiency in the range of 1-90% (v/v) to the total gas amount, preferably 10-85% (v/v). If the amount of carbon dioxide is used less than 1% (v/v), it is impossible to obtain the desired effect. On the other hand, if it is used more than 90% (v/v), it is impossible to perform smooth oxidation due to low concentration of oxygen. The carbon dioxide is applied periodically or continually in the vapor or liquid reaction medium. When it is applied into the vapor reaction medium, it is recycled after mixing with fresh gas or reaction waste containing remained oxygen and carbon dioxide.
Aromatic carboxylic acids of the present invention are prepared by batch or continuous process. Preferred temperature is in the range of 100-255xc2x0 C., more preferably 170-210xc2x0 C. If the temperature is lower than 100xc2x0 C., the reaction rate is too slow to be practical. On the other hand, if is higher than 255xc2x0 C., it is not economical due to side reactions. Reaction pressure is used to keep alkylaromatic compounds, its intermediates, and solvent in partially liquid state and preferably 1-35 atm of gauge pressure, more preferably 8-30 atm.